Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand or DOD. Operation using all of the engine cylinders is referred to as an activated mode. A deactivated mode refers to operation using less than all of the cylinders of the engine (one or more cylinders not active).
To smoothly transition between the activated and deactivated modes, the internal combustion engine must produce sufficient drive torque with a minimum of disturbances. Otherwise, the transition will not be transparent to the driver. In other words, excess torque will cause engine surge and insufficient torque will cause engine sag, which degrades the driving experience.
Conventional engine control systems have been somewhat successful in transitioning between the activated and deactivated modes in vehicles equipped with automatic transmissions. Torque converter slip algorithms are used to help smooth the transitions between DOD modes.
Engine control of vehicles equipped with manual transmissions is more challenging because the driver intent is unknown. Specifically, the control system does not have enough data to accurately determine if the driver is about to upshift and increase the load on the engine, downshift and decrease the load, or simply maintain the current gear. Due to this uncertainty, it is very difficult to determine if the engine may be placed in the deactivated mode.